The invention described herein relates generally to heat pumping and refrigerating engines and more particularly to acoustic cooling engines.
U.S. Pat. No. 4,489,553 to Wheatley et al. discloses an intrinsically irreversible heat engine. The engine is intrinsically irreversible because it uses heat transfer processes which are intrinsically irreversible in the thermodynamic sense, in contrast to a conventional heat engine which approaches optimum efficiency as the heat transfer processes become increasingly reversible. The intrinsically irreversible heat engine comprises a first thermodynamic medium, such as a fluid, and a second thermodynamic medium, such as a set of parallel plates, which are in imperfect thermal contact with each other and which bear a broken thermodynamic symmetry with respect to each other. U.S. Pat. No. 4,489,553 is expressly incorporated by reference herein for all that it teaches and is hereafter referred to as the '553 patent.
As a heat pump or refrigerator, the intrinsically irreversible heat engine includes a driver for effecting a reciprocal motion of the fluid at a frequency which is approximately inversely related to the thermal relaxation time of the fluid relative to the plates. This motion, together with the cyclic variation in temperature and pressure of the fluid, results in the pumping of heat along the plates and the concomitant generation of a temperature difference along the length of the plates.
The acoustic heat pumping engine disclosed in the '553 patent comprises a housing which can be either a straight, J-shaped or U-shaped tube. One end of the housing is capped and the other end is closed by a diaphragm and voice coil, which serve as an acoustic driver for generating an acoustic wave within the housing. The housing is filled with a compressible fluid, such as a gas, capable of supporting an acoustic standing wave. The plates are located within the housing near the capped end. Different parts of the plates receive heat at different rates from the gas moved therethrough during the time of increasing pressure of a wave cycle, and give up heat at different rates to the gas as the pressure of the gas decreases during the appropriate part of the wave cycle. The imperfect thermal contact between the gas and the plates results in a phase lag different from 90.degree. between the local gas temperature and its local velocity. As a result there is an acoustically stimulated heat pumping action which results in a temperature difference along the length of the plates. The ends of the plates nearest the driver become cold and the ends of the plates farthest from the driver become hot.
A major technical problem with the acoustic heat pumping engine disclosed in the '553 patent is that there is acoustically driven convective motion within the housing resulting in thermal communication between the cold ends of the plates and the ambient temperature environment at the driver end of the housing. This thermal communication limits the low temperature achievable at the cold ends of the plates, which have only been cooled to a temperature near 0.degree. C. in practice. It is therefore desirable to design an acoustic cooling engine capable of reaching lower temperatures. The '553 patent disclosed a quarter wavelength long resonant housing with plates located at the far end of the housing from the acoustic driver. Now we have discovered that an effectively half wavelength resonant pressure vessel is operable and that better performance is achieved with a thermodynamic element, such as a set of plates, located proximate the acoustic driver.